For the measurement of temperatures, especially with electrical or electronic instruments, various basic principles have been employed. For example, it is known to measure a temperature by detecting the potential differences or emf produced by a junction of dissimilar metallic and/or semiconductive elements forming a thermocouple or thermopile.
The output of such elements is a voltage which is a function of temperature and hence such temperature sensors can be used for a wide variety of temperature measuring and control purposes.
However, when such thermoelements are used at temperatures above about 1000.degree. C. or when they must be associated with measuring instruments which are to be located in an environment which is hazardous to the sensor, inaccurate results may be obtained, the systems using such elements may be destroyed and considerable error is encountered.
There has been developed a class of temperature measuring which utilizes somewhat different principles to provide an output representing the temperature and which avoids the disadvantages of these earlier systems.
One of these alternative systems is a noise-temperature thermometer or noise-temperature system.
This system utilizes a metallic strand, wire, coil or other elongated conductor which generates an electrical output because of the thermal agitation of electrical charges within the conductor. The output is a noise voltage and is produced in the electrical conductor by thermal agitation, i.e. an increase in the electrical noise is observed with increasing temperature.
Such thermally generated noise in an electrical conductor, also known as Johnson noise, can even be produced at conductor temperatures approaching 0.degree. K. at which thermocouples become inefficient, and at temperatures up to several hundred degrees K. or more, as long as the conductor retains its stability.
The available thermal-noise power is proportional to the absolute temperature over the frequency bandwidth over which the noise is measured. For a fixed bandwidth the available thermal-noise power can be measured in terms of the noise voltage and is proportional to absolute temperature.
For further details on the theory of such systems, the circuitry utilizing same, and the application of the principles of Johnson noise as applied to temperature measurement, reference can be made to U.S. Pat. Nos. 2,710,899, 2,728,835, 2,768,266, 2,884,786, 3,818,761, 3,890,841, 3,956,936, 3,966,500 and the patents, applications and publications cited or mentioned therein.
Reference may also be had to German patent document (Open Application--Offenlegungsschrift) No. 2,115,033 and German Pat. Nos. 2,263,469 and 2,347,765.
From the latter references it is known to provide the sensing element of a noise thermometer with a ceramic support or mounting element in which the metal conductor or wire is mounted and from the ends of which extend leads to the remote circuitry for detecting the noise voltage.
As the above-mentioned publications will also show, the junctions between the metal wire forming the noise resistance and the leads can be provided with thermocouples or other thermoelements defined between two dissimilar metals, each of which has a conductor leading from the sensing head or temperature probe. Thus the probe can comprise a temperature-sensing head at the free end thereof, containing the noise-resistance wire and the junctions and thermocouples, and a cable or like structure leading from this head in the opposite direction and provided with four conductors in two pairs, each pair being connected to a single junction.
The circuitry responsive to the signals delivered by these conductors is fully described in the aforementioned patents.
In the mechanical construction of the noise thermometer or sensor it is known to provide the noise resistance as a coil, meander or other configuration which will ensure a considerable length in a limited volume, e.g. by mounting the coil or meander on, or winding the wire in a number of passes around, a ceramic element disposed in the measuring head. Furthermore, it is known to pack the interior of the sensor with ceramic particles, inter alia, for greater stability.
Notwithstanding the use of a ceramic body to support the noise resistor and a packed construction of the probe, difficulties are encountered which, with experience, we have traced to an unsatisfactory insulation of the junctions from one another and from the electrical elements of the system. Furthermore, problems have been encountered in assembling the probes and in fabricating the units which have led to solutions which also tend to prevent satisfactory insulation and separation of the junctions.